One of the criteria for imaging systems is the amount of throughput that can be obtained; that is, the number of patients that can be processed and from which complete data can be obtained in a given amount of time. Any thing that can be done to increase thoroughput, that is decrease the time required to obtain data from the patient is a plus as long as the data that is acquired is useful and as long as there is not increased risk of any kind, or increased discomfort to the patient during the scan.
Presently, separate images of two different spectral components such as water and lipids within the patient are sometimes obtained. The separate images are important for diagnostic purposes; since they supply the user with chemical information in addition to the morphological or anatomical information of conventional imaging. Moreover by using an appropriate shift of one image with respect to the other the two images can be combined resulting in an image free of chemical shift artifacts. However, until the above mentioned invention to obtain the separate images at least two scans of the patient were required; i.e., two imaging cycles such as spin echo cycles had to be processed to obtain the two images.
A unique pair of interelated sequences to obtain information on water and/or lipids in a patient was described in an article appearing in Radiology, entitled "Simple Proton Spectroscopic Imaging" by W. T. Dixon (Vol. 153, 1984, pp 189-194). In that article a method for encoding spectroscopic information into clinical images is explained. The image produced differentiates between the water and fat intensities. The method requires using a normal spin echo sequence in which the Hahn and gradient echoes coincide. In addition each excitation is repeated with the Hahn echo shifted by an appropriate interval. More particularly, the 180 degree Rf pulse is shifted by a time T to shift the Hahn echo with respect to the gradient echo an amount sufficient to cause the chemical shift between the echoes of water and lipids to be 180 degrees out of phase at the gradient echo time. The image produced with the described sequence clearly indicates the differences between the signals due to water and the signals due to fat.
By obtaining normal spin echo derived image data in addition to obtaining modified spin echo image data, the two images can be constructed. Thus, the described method enables imaging two spectral components in a single image corrected for the chemical shift artifact or obtaining separate images of each of two spectral components.
A disadvantage of the described method is the amount of time required for obtaining the data for imaging. More particularly, two scans are required to obtain the two images. It is worth noting that even with two scans no inhomogeneity data is acquired by the Dixon method. Any reduction in this amount of time required to obtain the images of each of the two spectral components would be advantageous and a sought after goal.
An object of the invention of the above referred to patent application was to obtain separate data contributions from first and second spectral components sufficient to construct an image for each of the components with a single magnetic resonance scan; thus, cutting the scan time of Dixon by at least one half. The objective was obtained by causing there to be a 90 degree phase shift between the water and lipid in the time for acquiring the echo signal.
As used herein scan time is the time required to apply all of the excitation pulses and gradient pulses to enable acquiring sufficient data to construct an image of a selected volume of a sample being imaged. A single scan is the mininmal Rf signal transmitting and receiving repetitions required to acquire the data for an image of a single spectral component having the desired spatial resolution and signal-to-noise ratio.
The invention of the previous application and the improvement thereover of this application may be better understood when considering the mathematics of the data acquisition of the two spectral components.
In complex spectral notation: EQU Es=Ew+Ew+i El
where:
Es is the total echo signal received, PA1 Ew is the water component of the total echo signal, PA1 El is the lipid component of the total echo signal, and PA1 i is an operator indicating El is an imaginary number. PA1 Is is the total complex amplitude of the signal in each pixel, PA1 Iw is the water component of the total amplitude in each pixel, and PA1 Il is the lipid components of the total amplitude in each pixel. PA1 Io is the magnitude of the image per pixel (after the Fourier Transform) PA1 .psi. is the ideal phase angle between the water component and the vector resultant of the two spectral components of the signal in each pixel.
The complex notion transforms in the image (space) domain into: EQU ls=Iw+i Il
where:
In complex polar notation: EQU Is=Ioe.sup.-i.psi.
where:
According to the invention of the prior application the angle between the water and lipid components was set to be 90 degrees. To account for linear phase contributions and phase contributions due to inhomogeneities and the like, a calibration measurement was made pixel by pixel using a pure water phantom. Then the separate water and lipid contributions were computed.
The above inventive method works fine for many experiments, it however failed to account for phase delays due to inhomogeneities caused by the object being sampled. Thus, the inhomogeneity of the field Bo in the sample is different than the inhomogeneity of the field Bo in the water phantom. The problem was how to account for the phase delay .DELTA..phi. due to the inhomogeneity of the field Bo in the sample, where .DELTA..phi..beta. is the phase delay due to the total inhomogeneity.
According to a broad aspect of the present invention it is an object thereof to provide a method for determining a map of the local inhomogeneity within the sample.
Accordingly, it is an object of the present invention to acquire separate data contributions from first and second spectral components. The object includes having the data corrected for the phase delay .DELTA..phi. due to the inhomogeneity of the field within the sample as well as due to linear phase delays .beta.. The data may be acquired with a single scan and in sufficient quantities to construct an image of each of the spectral components, a combined image without chemical shift artifacts and/or a sample inhomogeneity map.